1. Field of the Invention
The invention relates generally to a continuous process for depositing layers onto small particles, and more particularly to a continuous process for atomic or molecular layer deposition onto small particles, in particular nanoparticles.
2. Description of the Related Art
Several techniques are known for depositing layers of a material onto a solid substrate. Examples include electroplating; electroless plating; chemical vapor deposition; and atomic or molecular layer deposition. The various techniques are essentially carried out in batch mode, and the deposition process may have to be repeated several times in order to obtain a coating of a specific desired thickness. As a result, the state of the art processes tend to be cumbersome and expensive.
GB 2 214 195 A discloses a pneumatic transport reactor for coating particles with a metal, for example Ni, Fe or Co, by decomposing the gaseous carbonyl of the metal thermally on the heated surface of the particles. The apparatus is constructed in the form of a loop, comprising a downwardly extending section and an upwardly extending section. The particles are mixed with a carrier gas containing the metal carbonyl in the downward section. The carbonyl is decomposed in the upward section, depositing metal on the particle. The apparatus comprises a separator, such as a cyclone, for separating the particles from the carrier gas.
The apparatus is suitable for particles having a particle size in the lower micron range, on the order of 4 μm. The particles may be circulated through the closed loop until the desired coating thickness is achieved, in what is essentially a batch-wise operation.
Puurunen, “Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process” Journal of Applied Physics 97, 121301 (2005) provides an overview of atomic layer deposition techniques in general, and of alumina in particular. In essence, atomic layer deposition (“ALD”) is a specific form of chemical vapor deposition, based on self-terminating gas-solid reactions.
The growth of layers by ALD consists of repeating reaction cycles consisting of four steps:
(1) A self-terminating reaction of a first reactant (Reactant A) with the surface of a solid substrate;
(2) A purge or evacuation to remove non-reacted Reactant A and any gaseous reaction by-products;
(3) A self-terminating reaction of a second reactant (Reactant B), or another treatment to activate the surface of the substrate again for a reaction with Reactant A.
(4) A purge or evacuation of excess Reactant B and of gaseous reaction products produced in step (3).
Step (1) is self-terminating in the sense that it stops when a monolayer is formed. A monolayer, in the context of ALD, is formed when all chemisorption sites available for Reactant A at the surface of the substrate are occupied. An important advantage of ALD is that layers are deposited epitaxially, resulting in a coating that is well defined down to an atomic scale. However, with ALD, by definition, only one atomic layer is deposited in each reaction cycle. For the formation of a relatively thick coating ALD may thus be less suitable, as the deposition of such coating may require tens, sometimes hundreds or even thousands of reaction cycles.
US Published Patent Application 2006/0062902 A1 discloses use of ALD for producing CIGS particles for use in photovoltaic panels. The particles are agitated to form a fluidized bed during coating, so that all the surface area of the suspended particle is accessible for surface reactions.
Helmsing et al., “Short Contact Time Experiments in a Novel Benchscale FCC Riser Reactor”, Chemical Engineering Science, Vol. 51, No. 11, pp 3039-3044 (1996) disclose an entrained flow reactor consisting essentially of a long, thin tube. The tube is looped so as to fit in a heating chamber of a manageable size. The reactor can be operated under plug flow conditions, making it suitable for testing catalysts used in fluid catalytic cracking (“FCC”) of crude oil fractions. The reactor has a single injection point for reactants.
Thus, there is a particular need for a continuous process for depositing atomic or molecular layers onto small particles, and for an apparatus for carrying out such a continuous process.